Miscible stimulation and flooding of petroliferous formations utilizing viscosified oil-based fluids

ABSTRACT

A viscosified miscible enhanced oil recovery fluid includes (a) a hydrocarbon fluid and viscosifying agent, wherein the viscosified fluid is a Newtonian fluid, (b) a hydrocarbon fluid and gelling agent, wherein the viscosified fluid is a power law fluid, (c) a hydrocarbon fluid, a gelling agent, and a rheological additive, wherein the viscosified fluid is a yield power law fluid, or (d) a hydrocarbon fluid, a gelling agent, a rheological additive, and solvent for the rheological additive, wherein the viscosified fluid is a yield power law fluid. The hydrocarbon fluid is preferably weighted with nano-scale weighting agents. The viscosified fluids are prepared and pumped into a subterranean petroliferous formation to recover a majority of the oil, heavy oil, condensate, or gas originally in place. A method for subsequently recovering a substantial fraction of the miscible injectant conventionally or by applying a viscosity breaker and recovering the viscosity-broken miscible injectant conventionally.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of and priority to U.S. Provisional Application Ser. No. 61/210,615 entitled “Miscible stimulation and flooding of petroliferous formations utilizing viscosified oil-based fluids” and filed Mar. 20, 2009, Confirmation No. 2199. Said provisional application is incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to viscosified, oil-based or hydrocarbon-based fluids and the use of said viscosified, oil-based or hydrocarbon-based fluids in miscible stimulation or miscible flooding. The invention relates further to the use of readily available oil-based or hydrocarbon-based fluids which may be converted into viscosified—Newtonian, Bingham-plastic, power-law, or yield-power-law—fluids for use in miscible stimulation or miscible flooding of subterranean petroliferous formations.

2. Background Art

Whereas typically processes which are termed primary production of subterranean petroliferous formations produce some 30% to 55% of the hydrocarbons originally in place in said formation, at least one miscible flooding process is known in the history of the Petroleum Industry which proved to be over 95% efficient. It was a very special miscible flood in which the conformation of the subterranean petroliferous formation worked in nearly perfect conjunction with the earth's gravity to retain the miscible drive fluid always above the oil in place, so a wellbore or collection of wellbores penetrating into the lowest reaches of the reservoir was able to efficiently recover the oil as the miscible fluid was being injected into the top the formation. After a suitable layer of miscible fluid had been placed in the top of the reservoir, injection was resumed but utilizing a gas that was miscible with the injectant that was miscible with the oil in place. This remarkable miscible flooding project has been well known to those skilled in the art for decades before the process of recovering oil from a gravity drainage oil-bearing formation described in McGuire, U.S. Pat. No. 5,720,350.

Many years ago, a passing thought of the inventor was to order up a sample of a suitable mixture of Arabian Light and Arabian Heavy crude—many billions of barrels of each was available at the time—as a miscible displacing agent for Prudhoe Bay crude, for no matter the cost, that would be the one of the best possible ways to accomplish that displacement. It was a flippant and irreverent thought which was never expressed out loud by the inventor, though in certain respects the thought was right—the mixture of Arabian Light and Arabian Heavy crudes could be injected low into the oil rim of the largely horizontally configured Prudhoe Bay field and would likely be much less mobile than the Prudhoe Bay crude. As a result, the miscible displacement should be among the more efficient such miscible floods known in the Petroleum Industry. Economically, of course, the idea was ludicrous because not even the relatively less costly Arabian Heavy crude will ever be readily available economically as a miscible injectant for the Prudhoe Bay field. Just transportation costs for getting the injectant to the field would raise the cost of oil production well beyond its market value during most of the life of the Prudhoe Bay field. Conceivably, there might one day be found a field where such a miscible injectant would be economically readily available for conducting such an efficient miscible flood of a subterranean petroliferous formation—maybe somewhere in Saudi Arabia, Arabian Light crude could be efficiently produced from a miscible drive due to the injection of Arabian Heavy crude.

Therefore, there exists a need to inject into the lower reaches of a largely horizontally disposed subterranean petroliferous formation—an oil, heavy oil, gas condensate, or gas field or a field comprising a mixture of these—of the general type such as, for example, the Prudhoe Bay field, a readily and economically available injectant—a drive fluid or injectant for miscible stimulation—that happens to be miscible with or multiple-contact-miscible with the oil, heavy oil, gas condensate, or gas in place in said largely horizontally disposed subterranean petroliferous formation but which also happens to be substantially lower in mobility than said oil, heavy oil, gas condensate, or gas in place so that the drive fluid will seldom by-pass any of the oil, heavy oil, gas condensate, or gas in place but will mostly confine it and keep it moving ahead of the injectant so that the oil, heavy oil, gas condensate, or gas in place is efficiently swept toward the producing well or wells where the oil, heavy oil, gas condensate, or gas formerly in place may be collected and transported from the wellbore to a suitable receiver, gathering center, or rail, ship, or pipeline conveyance wherewith the produced oil, heavy oil, gas condensate, or gas may be monetized.

As recognized by the present inventor in Leggett et al., U.S. Patent Application Serial No. US2007149412 (published Jun. 28, 2007 and naming the present inventor as a coinventor), but to meet altogether different performance objectives, a variety of fluids, such as packer fluids, have been developed in the past which incidentally also possess miscibility with or multiple-contact-miscibility with a wide range of oils, heavy oils, gas condensates, and/or gases in place in largely horizontally disposed subterranean petroliferous formations but which fluids developed in the past also happen to be substantially lower in mobility than said oil, heavy oil, gas condensate, or gas in place. Leggett et al. is incorporated herein by reference even though the present invention is directed at injection of a fluid into a petroliferous formation rather than into an annulus and even though the present disclosure improves upon Leggett et al. Hyde et al., U.S. Pat. No. 3,613,792 describes simple fluids which may be used as the injectant medium. Brandt et al., U.S. Pat. No. 4,258,791 improves on these injectant materials by disclosing an oleaginous liquid such as topped crude oils, gas oils, kerosene, diesel fluids, heavy alkylates, fractions of heavy alkylates, and the like in combination with an aqueous phase, lime, and a polymeric material. House, et al., U.S. Pat. No. 4,528,104 teaches a fluid comprised of an oleaginous liquid such as diesel oil, kerosene, fuel oil, lubricating oil fractions, heavy naphtha and the like in combination with an organophillic clay gellant and a clay dispersant such as a polar organic compound and a polyfunctional amino-silane.

Leggett et al. discloses a packer or annular fluid that includes a hydrocarbon fluid; and a gelling agent; wherein the packer fluid is a yield power law fluid. A method for preparing a packer fluid includes preparing a mixture of a hydrocarbon fluid, and a gelling agent; heating the mixture to a selected temperature; and shearing the mixture. A method for emplacing a packer fluid into an annulus includes preparing the packer fluid that includes a hydrocarbon fluid and a gelling agent, wherein the packer fluid is a yield power law fluid; and pumping the packer fluid into the annulus.

As also recognized by the present inventor in Leggett et al., gelled hydrocarbons have been successfully used as hydraulic fracturing fluids and viscosified fluids because the gel formation suitably increases the viscosities of the fluids. However, such fluids have not been used as miscible injectants into a petroliferous formation. As further recognized by the present inventor in Leggett et al., polyvalent metal (typically, ferric iron or aluminum or the chelated forms of ferric iron or aluminum) salts of phosphoric acid esters have been successfully used as gelling agents for forming high viscosity gelled hydrocarbon fluids. Description of these fluids and their uses can be found in U.S. Pat. Nos. 4,507,213 issued to Daccord et al., 4,622,155 issued to Harris et al., 5,190,675 issued to Gross, and 5,846,915 issued to Smith et al. More recently, U.S. Pat. No. 6,511,944 issued to Taylor et al. discloses gelled hydrocarbon hydraulic fracturing fluids that include ferric iron or aluminum polyvalent metal salts of phosphonic acid esters, instead of phosphoric acid esters. These gelled hydrocarbon fracture fluids are power law fluids rather than yield power law fluids, i.e., they exhibit no yield stress: τ_(y)=0. These patents are hereby incorporated herein by reference even though the present invention improves upon them, among other ways, by teaching how to make miscible enhanced oil recovery fluids into yield power law fluids, i.e., those that exhibit τ_(y)≠0.

Additionally, as recognized by the present inventor in Leggett et al., another short-coming of the above-referenced hydraulic fracturing fluids has been their limited stability—after all, they need only last a matter of hours, since even a massive hydraulic fracturing job involving 2,000,000 pounds of proppant is typically concluded in less than 8 hours. Although these fluids have worked well in the hydraulic fracturing application, Leggett et al. describes that there is still a need for insulating annular or packer fluids that are stable for extended periods, low in thermal conductivity, and simultaneously inhibitive of convective heat loss. In the present invention, there is still a need for miscible injectant fluids that are stable for extended periods.

SUMMARY OF INVENTION

The present invention is directed to a method for injecting a viscosified fluid into a subterranean petroliferous formation and includes preparing the viscosified fluid that includes (a) a hydrocarbon fluid or weighted hydrocarbon fluid and a viscosifying agent, wherein the viscosified fluid is a Newtonian fluid, (b) a hydrocarbon fluid or weighted hydrocarbon fluid and a gelling agent, wherein the viscosified fluid is a power law fluid, (c) a hydrocarbon fluid or weighted hydrocarbon fluid, a gelling agent, and a rheological additive, wherein the viscosified fluid is a yield power law fluid, or (d) a hydrocarbon fluid or weighted hydrocarbon fluid, a gelling agent, a rheological additive, and a solvent for the rheological additive, wherein the viscosified fluid is a yield power law fluid; and pumping the viscosified fluid into the subterranean petroliferous formation in order to recover a majority of the oil, heavy oil, condensate, or gas originally in place. A method for subsequently recovering a substantial fraction of the miscible injectant conventionally or by applying a viscosity breaker and recovering the viscosity-broken miscible injectant conventionally is also disclosed along with the exemplary viscosified fluids and the methods of making these fluids.

In one embodiment of the present invention there is disclosed a viscosified fluid comprising a weighted hydrocarbon fluid and a viscosifying agent. In another embodiment of the present invention there is disclosed a viscosified fluid comprising a weighted hydrocarbon fluid and a gelling agent. In yet another embodiment of the present invention there is disclosed a viscosified fluid comprising a weighted hydrocarbon fluid, a gelling agent and a rheological additive. The hydrocarbon fluid may comprise at least one selected from diesel, a mixture of diesels and paraffin oil, mineral oil, and isomerized olefins. The weighted viscosified fluids of the present invention are designed to exhibit the fluid characteristics or properties of power law fluids, yield power law fluids, Newtonian fluids or Bingham-plastic fluids. In one embodiment, the weighted hydrocarbon fluid comprises a nano-scale dispersion of a weighting agent comprising at least one of barium sulfate, barium carbonate, zinc oxide, zinc nitride, magnesium oxide, or iron oxide, in a hydrocarbon-based fluid. In another embodiment, the weighted hydrocarbon fluid comprises a nano-scale-zinc-oxide dispersed in a hydrocarbon-based fluid. The gelling agent may comprise a multivalent metal ion and at least one ester selected from the group consisting of a phosphoric acid ester and a phosphonic acid ester. In one embodiment, the multivalent metal ion is at least one selected from the group consisting of a ferric ion, an aluminum ion, a chelated ferric ion and a chelated aluminum ion. The rheological additive may an alkyl diamide having a formula: R₁—HN—CO—(CH₂)_(n)—CO—NH—R₂, wherein n is an integer from 1 to 20, R₁ is an alkyl groups having from 1 to 20 carbons, and R₂ is hydrogen or an alkyl group having from 1 to 20 carbons. In one embodiment, the rheological additive is present at a concentration of 3-13 pounds per barrel.

There is also disclosed a method for preparing a viscosified fluid as described herein, comprising the steps of preparing a mixture of a hydrocarbon fluid (such as that described herein) and a viscosifying agent; optionally heating the mixture to a selected temperature or not; and optionally mixing or shearing the mixture. Another method for preparing a viscosified fluid (as described herein) comprises the steps of preparing a mixture of a desired hydrocarbon fluid and a desired gelling agent (such as those described herein); optionally heating the mixture to a selected temperature or not; and optionally mixing or shearing the mixture. Yet another method for preparing a viscosified fluid comprises the steps of preparing a mixture of a desired hydrocarbon fluid (as described herein), a desired gelling agent (e.g., as described herein), and a desired rheological additive (as described herein); heating the mixture to a selected temperature; and shearing the mixture. Another method for preparing a viscosified fluid comprises the steps of preparing a mixture of a desired hydrocarbon fluid and a desired rheological additive, as described herein; heating the mixture to a selected temperature; shearing the mixture; and adding in a desired gelling agent. In another method for preparing a viscosified fluid, the steps comprise preparing a mixture of a desired hydrocarbon fluid, a desired gelling agent, a desired rheological additive, and a solvent for the rheological additive; heating the mixture to a selected temperature; and shearing the mixture. Another method disclosed for preparing a viscosified fluid comprises the steps of preparing a mixture of a hydrocarbon fluid, a rheological additive, and a solvent for said rheological additive; heating the mixture to a selected temperature; shearing the mixture; and adding in a gelling agent.

There is also disclosed a method for injecting a viscosified fluid into a subterranean petroliferous formation, comprising the steps of preparing the desired viscosified fluid as described herein (designed to exhibit the fluid characteristics or properties of power law fluids, yield power law fluids, Newtonian fluids or Bingham-plastic fluids) and pumping the viscosified fluid into the subterranean petroliferous formation. In one embodiment of this method, the viscosified fluid comprises a hydrocarbon fluid and a viscosifying agent and is a Newtonian fluid. In another embodiment of this method, the viscosified fluid comprises a weighted hydrocarbon fluid (as described herein) and a viscosifying agent and is a Newtonian fluid. In yet another embodiment of this method, the viscosified fluid comprises a hydrocarbon fluid and a gelling agent and is a power law fluid. In another embodiment of this method, the viscosified fluid comprises a weighted hydrocarbon fluid (as described herein) and a gelling agent and is a power law fluid. In yet another embodiment of this method, the viscosified fluid comprises a hydrocarbon fluid, a gelling agent, and a rheological additive (and optionally, the addition of a solvent for the rheological additive) and is a yield power law fluid. In yet another embodiment of this method, the viscosified fluid comprises a weighted hydrocarbon fluid (as described herein), a gelling agent, and a rheological additive (and optionally, the addition of a solvent for the rheological additive) and is a yield power law fluid. These methods may further comprise the step of subsequently recovering some (preferably a substantial fraction) of the viscosified fluid by applying a viscosity breaker proximate the viscosified fluid in the subterranean petroliferous formation and then flowing the viscosity-broken fluid back to the surface. Additionally, these methods of injection may be preceded, if desired, by the initial step of determining whether alkalinity conditions exist in the petroliferous formation that could be damaging to the viscosified fluid, and if so, prior to the injection of the viscosified fluid, injecting a mild acid or acid gas to neutralize the alkalinity.

Also disclosed is a viscosified miscible enhanced oil recovery fluid for injection into a subterranean petroliferous formation comprising: a weighted hydrocarbon-based fluid comprising a nano-scale weighting agent dispersed in a hydrocarbon-based fluid, and at least one additive selected from the group consisting of viscosifying agents, gelling agents and rheological agents, wherein the viscosified miscible enhanced oil recovery fluid has fluid behavior characteristics selected from the group consisting of yield power law fluids, power law fluids, Bingham-plastic fluids and Newtonian fluids. The nano-scale weighting agent may comprise at least one of barium sulfate, barium carbonate, zinc oxide, zinc nitride, magnesium oxide, or iron oxide. The nano-scale weighting agent may comprise a nano-scale-zinc-oxide. The gelling agent may comprise a multivalent metal ion and at least one ester selected from the group consisting of a phosphoric acid ester and a phosphonic acid ester. In one embodiment, the multivalent metal ion is at least one selected from the group consisting of a ferric ion, an aluminum ion, a chelated ferric ion and a chelated aluminum ion. The rheological agent may be an alkyl diamide having a formula: R₁—HN—CO—(CH₂)_(n)—CO—NH—R₂, wherein n is an integer from 1 to 20, R₁ is an alkyl groups having from 1 to 20 carbons, and R₂ is hydrogen or an alkyl group having from 1 to 20 carbons. In one embodiment, the rheological agent is present at a concentration of 3-13 pounds per barrel. The rheological agent may also be used with a solvent for the rheological agent. The hydrocarbon-based fluid can comprise at least one selected from diesel, a mixture of diesels and paraffin oil, mineral oil, and isomerized olefins. In one embodiment, the additive comprises a viscosifying agent. In one embodiment, the additive comprises a gelling agent. In one embodiment, the additive comprises a gelling agent and a rheological agent. In one embodiment, the additive comprises a gelling agent, a rheological agent and a solvent for the rheological agent.

There is also disclosed a method for preparing a viscosified miscible enhanced oil recovery fluid for injection into a subterranean petroliferous formation comprising the steps of: preparing a mixture of a weighted hydrocarbon-based fluid comprising a nano-scale weighting agent dispersed in a hydrocarbon-based fluid and at least one additive selected from the group consisting of viscosifying agents, gelling agents and rheological agents; optionally heating the mixture to a selected temperature; and optionally shearing the mixture, wherein the mixture has fluid behavior characteristics selected from the group consisting of yield power law fluids, power law fluids, Bingham-plastic fluids and Newtonian fluids.

Additionally, there is disclosed a method for enhanced oil recovery from a subterranean petroliferous formation comprising the steps of: preparing a viscosified miscible enhanced oil recovery fluid comprising: a mixture of (a) a weighted hydrocarbon-based fluid comprising a nano-scale weighting agent dispersed in said hydrocarbon-based fluid, and (b) at least one additive selected from the group consisting of viscosifying agents, gelling agents and rheological agents; and pumping said fluid mixture from the surface into said subterranean petroliferous formation; wherein said viscosified miscible enhanced oil recovery fluid exhibits fluid behavior characteristics selected from the group consisting of yield power law fluid characteristics, power law fluid characteristics, Bingham-plastic fluid characteristics, and Newtonian fluid characteristics. In one embodiment of this method, the nano-scale weighting agent comprises at least one of barium sulfate, barium carbonate, zinc oxide, zinc nitride, magnesium oxide, or iron oxide. In another embodiment, the nano-scale weighting agent comprises a nano-scale-zinc-oxide. The gelling agent may comprise a multivalent metal ion and at least one ester selected from the group consisting of a phosphoric acid ester and a phosphonic acid ester. In one embodiment, the multivalent metal ion is at least one selected from the group consisting of a ferric ion, an aluminum ion, a chelated ferric ion and a chelated aluminum ion. The rheological additive used in this method may be an alkyl diamide having a formula: R₁—HN—CO—(CH₂)_(n)—CO—NH—R₂, wherein n is an integer from 1 to 20, R₁ is an alkyl groups having from 1 to 20 carbons, and R₂ is hydrogen or an alkyl group having from 1 to 20 carbons. The rheological additive in one embodiment is present at a concentration of 3-13 pounds per barrel. A solvent can be employed for the rheological agent.

In the practice of this method, the hydrocarbon fluid may comprise at least one selected from diesel, a mixture of diesels and paraffin oil, mineral oil, and isomerized olefins. In one embodiment of this method, the additive comprises a viscosifying agent. In another embodiment of this method, the additive comprises a gelling agent. In yet another embodiment of this method, the additive comprises a gelling agent and a rheological agent. In a further embodiment of this method, the additive comprises a gelling agent, a rheological agent and a solvent for the rheological agent. The method may further comprise the step of subsequently recovering some (but preferably a substantial fraction) of the viscosified miscible enhanced oil recovery fluid back to the surface from the subterranean petroliferous formation. The method may further comprise the step of subsequently recovering some of the viscosified miscible enhanced oil recovery fluid by first applying a viscosity breaker proximate to the viscosified miscible enhanced oil recovery fluid in the subterranean petroliferous formation and then flowing some (but preferably a substantial fraction) of the viscosity-broken fluid back to the surface. The method may further comprise the initial step of determining whether alkalinity conditions exist in the petroliferous formation that could be damaging to the viscosified miscible enhanced oil recovery fluid, and if so, prior to the injection of the miscible viscosified enhanced oil recovery fluid, a mild acid or acid gas is injected to neutralize the alkalinity.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

As recognized by the present inventor in Leggett et al., FIG. 1 shows a chart of shear stress vs. shear rate for a yield power law viscosified injectant fluid in accordance with one embodiment of the invention. This FIGURE is illustrative of the behavior of a yield power law fluid. However, if a similar chart were to show a shear stress vs. shear rate curve for a Newtonian viscosified injectant fluid in accordance with another embodiment of the invention, there would not be a curve as in FIG. 1, but instead a straight line with zero slope. If a similar chart were to show a shear stress vs. shear rate curve for a power law viscosified injectant fluid in accordance with yet another embodiment of the invention, there would not be a curve as in FIG. 1, but instead a straight line with non-zero slope.

DETAILED DESCRIPTION OF INVENTION

A first embodiment in accordance with the present invention is to inject into the lower reaches of a largely horizontally disposed subterranean petroliferous formation—an oil, heavy oil, gas condensate, or gas field or a field comprising a mixture of these—of the general type such as, for example, the Prudhoe Bay field, a readily and economically available injectant—a drive fluid or injectant for miscible stimulation—that happens to be miscible with or multiple-contact-miscible with the oil, heavy oil, gas condensate, or gas in place in said largely horizontally disposed subterranean petroliferous formation but which also happens to be substantially lower in mobility than said oil, heavy oil, gas condensate, or gas in place so that the drive fluid will seldom by-pass any of the oil, heavy oil, gas condensate, or gas in place but will mostly confine it and keep it moving ahead of the injectant so that the oil, heavy oil, gas condensate, or gas in place is efficiently swept toward the producing well or wells where the oil, heavy oil, gas condensate, or gas formerly in place may be collected and transported from the wellbore to a suitable receiver, gathering center, or rail, ship, or pipeline conveyance wherewith the produced oil, heavy oil, gas condensate, or gas may be monetized.

An optional variant to the first embodiment in accordance with the present invention involves a novel nano-scale-zinc-oxide dispersed in a hydrocarbon-based fluid available from Liquid Minerals Group, Inc. (New Waverly, Tex.) to create a weighted hydrocarbon-based fluid. Alternative weighting agents for such nano-scale dispersed weighted hydrocarbon-based fluids runs the usual gamut of weighting agents for drilling fluids—weighting agents such as, for example, barium sulfate, barium carbonate, zinc oxide, zinc nitride, magnesium oxide, iron oxide, and mixtures thereof. These weighting agents can be prepared using nanotechnology and blended into a hydrocarbon-based fluid to create a nano-scale dispersed weighted fluid. This fluid can be blended with hydrocarbon-based injectant fluids to achieve a Newtonian miscible injectant of any density between that of the ordinary miscible injectant (˜5 to ˜7.2 lb_(m)/gal) and the hydrocarbon-based fluid involving the dispersed nano-scale zinc oxide (˜11.5 lb_(m)/gal). If the largely horizontally configured subterranean petroliferous formation has even the slightest deviation from horizontal, then a denser (˜9 to ˜11 lb_(m)/gal) injectant can be injected low in the formation on the end of the reservoir toward the lower end and the injectant will tend to remain in the lower reaches of the formation in relationship to the less dense (˜5 to ˜9 lb_(m)/gal) original oil, heavy oil, gas condensate, or gas in place, permitting the latter to be produced from a well or wells completed in the upper reaches of the upper end of the reservoir.

A second embodiment in accordance with the present invention begins much as the first embodiment in accordance with the present invention but at the end of production of the original oil, heavy oil, gas condensate, or gas in place, subsequent steps are performed by novel means—such as, for example, employing a viscosity breaker to the injectant—or by means already well known to those skilled in the art—such as, for example, employing a gas or steam stimulation, gas flood, steam flood, or waterflood—so that the larger part of the injectant may also be recovered from the formation in addition to virtually all of the original oil, heavy oil, gas condensate, or gas in place.

A third embodiment in accordance with the present invention is to first obtain a readily and economically available injectant that happens to be miscible with or multiple-contact-miscible with the oil, heavy oil, gas condensate, or gas in place in said largely horizontally disposed subterranean petroliferous formation and treat it by novel means or by means already well known to those skilled in the art so that said available injectant relatively inexpensively acquires Newtonian, Bingham plastic, power law, or yield power law (Herschel-Bulkley) rheological properties such that it becomes a readily and economically available injectant which also happens to be substantially lower in mobility than said oil, heavy oil, gas condensate, or gas in place. Thereafter, the third embodiment in accordance with the present invention proceeds just as the first embodiment in accordance with the present invention: to inject into the lower reaches of a largely horizontally disposed subterranean petroliferous formation—an oil, heavy oil, gas condensate, or gas field or a field comprising a mixture of these—of the general type such as, for example, the Prudhoe Bay field, a readily and economically available injectant—a drive fluid or injectant for miscible stimulation—that happens to be miscible with or multiple-contact-miscible with the oil, heavy oil, gas condensate, or gas in place in said largely horizontally disposed subterranean petroliferous formation but which also happens to be substantially lower in mobility than said oil, heavy oil, gas condensate, or gas in place so that the drive fluid will seldom by-pass any of the oil, heavy oil, gas condensate, or gas in place but will mostly confine it and keep it moving ahead of the injectant so that the oil, heavy oil, gas condensate, or gas in place is efficiently swept toward the producing well or wells where the oil, heavy oil, gas condensate, or gas formerly in place may be collected and transported from the wellbore to a suitable receiver, gathering center, or rail, ship, or pipeline conveyance wherewith the produced oil, heavy oil, gas condensate, or gas may be monetized.

The teachings of Leggett et al. focus on the use of yield power fluids (non-Newtonian fluids) as packer fluids, and provide that for an insulating annular fluid, rheological behavior is needed that is different from the power law, in other words, for these insulating annular fluids, what is needed is yield power behavior. Additionally, the teachings of Leggett et al. would provide against the use of any weighted fluid inasmuch as while the weighting agent remained dispersed throughout the fluid, it would increase the thermal conductivity (exactly what Leggett et al. was trying to avoid) and inasmuch as conventional weighting agents are only micronized or even coarser and, as such, cannot be maintained in dispersion for extended periods like 5 years or 40 years. In contrast, with the present invention, the weighting agents are nano-scale, therefore they remain dispersed indefinitely through the action of Brownian motion and the increased thermal conductivity is not a detriment. As such, the viscosified miscible enhanced oil recovery fluid of the present invention can utilize these nano-scale weighting agents to advantage in yield power law fluids (such as those described in Leggett et al.), power law fluids, Bingham-plastic fluids, and Newtonian fluids.

Creating a weighted viscosified miscible enhanced oil recovery fluid of the present invention can begin with a base fluid containing these nano-scale weighting agents and the base fluid can subsequently be converted to yield power law fluids, power law fluids, Bingham-plastic fluids, or Newtonian fluids depending on the nature of the viscosifying agents or gelling agents added subsequently to the base fluid containing these nano-scale weighting agents. The viscosifying agents or gelling agents may be conventional viscosifying agents or gelling agents well known to those of skill in the art or may be viscosifying agents or gelling agents taught in Leggett et al., or viscosifying agents or gelling agents taught herein.

A fourth embodiment in accordance with the present invention begins much as the third embodiment in accordance with the present invention but at the end of production of the original oil, heavy oil, gas condensate, or gas in place, subsequent steps are performed by novel means—such as, for example, employing a viscosity breaker to the injectant—or by means already well known to those skilled in the art—such as, for example, employing a steam stimulation, steam flood, or waterflood—so that the larger part of the injectant may also be recovered from the formation in addition to virtually all of the original oil, heavy oil, gas condensate, or gas in place.

In one aspect, the present invention also relates to viscosified, oil-based or hydrocarbon-based fluids and the use of said viscosified, oil-based or hydrocarbon-based fluids in miscible stimulation or miscible flooding. A miscible stimulation or miscible flooding fluid, (also referred to as a miscible enhanced oil recovery fluid) in accordance with one embodiment of the invention includes a hydrocarbon fluid, wherein the fluid is viscous, Newtonian fluid. In another aspect, the present invention relates to the use of readily available oil-based or hydrocarbon-based fluids which may be converted into viscosified—Newtonian, Bingham-plastic, power-law, or yield-power-law—fluids for use in miscible stimulation or miscible flooding of subterranean petroliferous formations. A miscible enhanced oil recovery fluid in accordance with one embodiment of the invention includes a hydrocarbon fluid and a gelling agent, wherein the fluid is a power law fluid. A miscible enhanced oil recovery fluid in accordance with another embodiment of the invention includes a hydrocarbon fluid; a gelling agent; and a rheological additive, wherein the fluid is a yield power law fluid. In one embodiment, exemplary yield power law insulating packer fluids of Leggett et al., could be employed to advantage as miscible enhanced oil recovery fluids. In other embodiments, the fluids used as miscible enhanced oil recovery fluids could be power law or Newtonian fluids which are not in accordance with the teachings of Leggett et al.

In another aspect, the present invention relates to methods for preparing a miscible injectant fluid. A method in accordance with one embodiment of the invention includes preparing a mixture of a hydrocarbon fluid and a gelling agent; and mixing the two without heating or with heating to a selected temperature. A method in accordance with another embodiment of the invention includes preparing a mixture of a hydrocarbon fluid, a gelling agent, and a rheological additive; heating the mixture to a selected temperature; and shearing the mixture. In another embodiment, the hydrocarbon fluid is a weighted hydrocarbon-based fluid where the weighting agent can include nano-scale-zinc oxide products.

In another aspect, the present invention relates to methods for injecting a viscosified fluid into a petroliferous formation. A method in accordance with one embodiment of the invention includes preparing the viscosified fluid that includes a hydrocarbon fluid and a viscosifying agent, wherein the viscosified fluid is a Newtonian fluid; and pumping the viscosified fluid into one or more injection well(s). Another method in accordance with one embodiment of the invention includes preparing the viscosified fluid that includes a hydrocarbon fluid and a gelling agent, wherein the viscosified fluid is a power law fluid; and pumping the viscosified fluid into one or more injection well(s). Yet another method in accordance with one embodiment of the invention includes preparing the viscosified fluid that includes a hydrocarbon fluid, a gelling agent, and a rheological additive, wherein the viscosified fluid is a yield power law fluid; and pumping the viscosified fluid into one or more injection well(s). In another embodiment, the hydrocarbon fluid is a weighted hydrocarbon-based fluid where the weighting agent can include nano-scale weighting products such as nano-scale-zinc oxide products. There is a degree of interchangeability between the terms “viscosifying agent” and “gelling agent”; and a gelled fluid would also be a viscosified fluid; but a viscosified fluid might not also be considered a gelled fluid; in the common usage if one had a Newtonian viscosified fluid and wanted to add into it a gelling agent, it would be understood that the Newtonian viscosified fluid would be turned into power law or yield power law fluid. Similarly, in the common usage if one had a power law viscosified fluid and wanted to add into it a gelling agent, it would be understood that the power law viscosified fluid would be turned into yield power law fluid; and if one had a Bingham plastic viscosified fluid and wanted to add into it a gelling agent, it would be understood that the Bingham plastic viscosified fluid would be turned into yield power law fluid.

Embodiments of the present invention relate to viscosified miscible enhanced oil recovery fluids and methods of preparing and injecting such fluids. Viscosified miscible enhanced oil recovery fluids according to the present invention have good long-term hydrocarbon-displacement properties, because they are miscible or multiple-contact miscible with a wide range of hydrocarbons originally in place in known subterranean petroliferous formations or those likely to be production targets in the foreseeable future. Viscosified miscible enhanced oil recovery fluids according to the present invention also resist syneresis and separation of various components into separate phases, and often have unique rheological properties that minimize their movement once they are injected—and this minimization of movement, in turn, helps keep the drive or stimulation fluid behind the hydrocarbons originally in place so that the drive fluid will seldom by-pass any of those hydrocarbons in place but will mostly confine them and keep them moving ahead of the injectant so that the hydrocarbons in place are efficiently swept toward the producing well or wells.

As per Leggett et al., gelled hydrocarbons have been successfully used as hydraulic fracturing fluids, as described in a number of patents and publications, such as U.S. Pat. Nos. 3,757,864 issued to Crawford et al., 4,104,173 issued to Gay et al., 4,200,539 issued to Burnham et al. and 4,507,213 issued to Daccord et al. These patents are incorporated by reference in their entireties. In fracturing fluids, high viscosity is important for suspending the proppants. On the other hand, it is undesirable because fracturing fluids need to be pumped very rapidly into the well and the fractures. In contrast, according to Leggett et al., minimization or elimination of fluid movement is highly desirable for packer fluids once they are emplaced in the annulus. In further contrast, in the present invention, minimization of fluid movement is highly desirable for miscible or multiple-contact-miscible drive or stimulation fluids once they are injected into the subterranean petroliferous formation.

Similar to the teachings of Leggett et al. relating to packer fluids, the viscosified miscible enhanced oil recovery fluids in accordance with embodiments of the invention are gelled oil-based (hydrocarbon-based) fluids having yield power law (Herschel-Bulkley) characteristics. Yield power law fluids have complex non-Newtonian rheological behavior. A yield power fluid does not start to move until an applied stress (force) exceeds its yield stress. Thus, a yield power law viscosified fluid will remain in largely place (i.e., not be prone to much movement) once it is injected into the producing formation. On the other hand, yield power law fluids tend to have relatively low high-shear-rate viscosity, making them easier to inject and, given a pumping shear stress in excess of the yield stress, to displace. That is, yield power law fluids can be pumped with relative ease into producing formations during emplacement, as long as the applied stress from pumping exceeds the yield stress. Then, once in place the shear stress decreases and the high low-shear-rate viscosity, makes them substantially lower in mobility than the oil, heavy oil, gas condensate, or gas in place so that the drive fluid will seldom by-pass any of the oil, heavy oil, gas condensate, or gas in place but will mostly confine it and keep it moving ahead of the injectant so that the oil, heavy oil, gas condensate, or gas in place is efficiently swept toward the producing well or wells where the oil, heavy oil, gas condensate, or gas formerly in place may be collected and transported from the wellbore to a suitable receiver, gathering center, or rail, ship, or pipeline conveyance wherewith the produced oil, heavy oil, gas condensate, or gas may be monetized. For a discussion of tools for analyzing yield power law fluids, see the article coauthored by the inventor, Horton, et al., “A New Yield Power Law Analysis Tool Improves Insulating Annular Fluid Design,” paper No. AADE-05NTCE-49, AADE 2005 National Technical Conference and Exhibit, Houston, Tex., Apr. 5-7, 2005, which is herein incorporated by reference.

As mentioned above and discussed in Leggett et al., gelled hydrocarbons have long been successfully used as hydraulic fracturing fluids. In fracturing fluids, the characteristic of high viscosity is important for suspending the proppants but high mobility is also needed for getting the proppant slurry down the well and out into the fracture. These somewhat contradictory objectives can be achieved by way of a shear-dependent viscosity, such as that characterized by the Power Law, equation 1:

τ=K·{dot over (γ)}^(m) ^(m)   (1)

where

-   -   τ is the shear stress (lb_(f)/100 ft²),     -   K is the consistency factor,     -   {dot over (γ)} is the shear rate (s⁻¹), and     -   n_(m) is the flow behavior index.         Hydraulic fracturing fluids are typically selected such that         they exhibit a flow behavior indices in the 0.5 to 0.8 range and         a suitable value of the consistency factor so that they will be         sufficiently viscous at moderate shear rate to carry proppant         efficiently and also sufficiently mobile at high shear rate to         allow the proppant slurry to move readily down the well and out         into the fracture. However, hydraulic fracturing fluids seldom         encounter the low shear rate range that viscosified miscible         enhanced oil recovery fluids (or the insulating packer fluids of         Leggett et al.) experience most of the time. For the latter,         rheological behavior is needed that is different from the power         law behavior, especially in the 0.3 to 0.003 sec⁻¹ shear rate         range (see paper No. AADE-05-NTCE-49 by Horton, et al.,         mentioned above). For these miscible enhanced oil recovery         viscosified fluids (or the insulating packer fluids of Leggett         et al.), what is needed is not only a somewhat lower flow         behavior index (preferably in the 0.4 to 0.7 range), but also a         relatively large value of the yield stress (also referred to as         τ_(y)), in the range of 10 to 105 lb_(f)/100 ft² as given in the         Yield Power Law Equation (also known as the Herschel-Bulkley         Equation), which is as follows:

τ=τ_(y) +K _(m)·{dot over (γ)}^(n) ^(m)   (2)

where

-   -   τ is the shear stress as in Equation 1,     -   τ_(y) is the yield stress (lb_(f)/100 ft²),     -   K_(m) is the consistency factor,     -   {dot over (γ)} is the shear rate (s⁻¹), and     -   n_(m) is the flow behavior index.         The shear rate environment of working viscosified miscible         enhanced oil recovery fluids (or the insulating packer fluids of         Leggett et al.) is such that, while the fluid is being emplaced         or displaced, τ_(y) in the range of 10 to 105 lb_(f)/100 ft² is         relatively unimportant compared with the other parameters given         in Equation 2; but the converse is true for the majority of the         lifetime of a working viscosified fluid (or the insulating         packer fluids of Leggett et al.)—here, the extended period of         time between injection and arrival at a producing well. This         latter fact is the reason why a conventional hydraulic         fracturing fluid is generally not best suited for use as a         viscosified miscible injectant fluid (or as an insulating packer         fluid per Leggett et al.).

In accordance with some embodiments of the invention, viscosified miscible injectant fluids or multiple-contact-miscible injectant fluids (much like the insulating packer fluids of Leggett et al.) may be based on conventional gelled hydrocarbons, but further include rheological additives to produce yield power law fluids. Conventional gelled hydrocarbons can be obtained by introducing phosphoric acid esters and an aluminum (or ferric) compound into hydrocarbon base fluids. These gelled hydrocarbon fluids have a three-dimensional polymer element in the hydrocarbons. The three-dimensional polymer element causing the gelling is constituted by phosphoric acid esters bonded (complexed) with aluminum or ferric cations. The presence of long alkyl side chains on the phosphoric acid ester render these polymer elements soluble in the hydrocarbons.

However, these conventional gelled hydrocarbon fluids are power law fluids or Newtonian law fluids; these fluids will move in response to any exerted force, including a small force. In contrast, viscosified miscible enhanced oil recovery fluids of the present invention (or the insulating packer fluids of Leggett et al.) include rheological additives that change these fluids from power law fluids to yield power law fluids. Yield power law fluids will not move until the exerted stress (force) exceeds the yield stress of the fluids. As noted above, yield power fluids are more preferred as viscosified miscible enhanced oil recovery fluids (or as insulating packer fluids of Leggett et al.) because they will not move in response to minor stress (e.g., vibration) from various activities in the well.

A viscosified miscible enhanced oil recovery fluid in accordance with embodiments of the invention (or the insulating packer fluids of Leggett et al.) comprises hydrocarbon base fluids, a gelling agent, and a rheological additive that makes the gelled hydrocarbons behave like a yield power law fluid. One of ordinary skill in the art would appreciate that various rheological additives may be used to impart a fluid with the desired yield power law characteristics. As identified in Leggett et al., suitable rheological additives in accordance with embodiments of the invention, for example, may include alkyl diamides, such as those having a general formula: R₁—HN—CO—(CH₂)_(n)—CO—NH—R₂, wherein n is an integer from 1 to 20, more preferably from 1 to 4, yet more preferably from 1 to 2, and R₁ is an alkyl groups having from 1 to 20 carbons, more preferably from 4 to 12 carbons, and yet more preferably from 5 to 8 carbons, and R₂ is hydrogen or an alkyl group having from 1 to 20 carbons, or more preferably is hydrogen or an alkyl group having from 1 to 4 carbons, wherein R₁ and R₂ may or may not be identical. Such alkyl diamides may be obtained, for example, from M-I L.L.C. (Houston, Tex.) under the trade name of VersaPac™.

The VersaPac™ product has been used as a thermally activated gelling agent, which generates viscosity and develops gel structure when sheared and heated to a temperature above 60° C. When the VersaPac™ product is fully activated, the gel structure remains stable even if the temperature drops below 60° C. However, when used at a temperature above its melting point (120° C.), the rheological effect gradually decreases.

The VersaPac™ product is activated by a combination of heat and shear. In the absence of shear and below the temperature of activation, the rheological effect of the VersaPac™ product is minimal because the particles do not swell. The gelling mechanism involves the swelling of the initial agglomerates and a gradual release of individual oligomer chains. The released oligomers then associate with other particulate material to produce the rheological effect. The build-up of this structure is thixotropic as it involves re-alignment of the initial structure to the most thermodynamically stable configuration. When totally activated, a type of micelle structure is formed involving the gelling agent and the other components in the system.

In accordance with embodiments of the invention, a viscosified miscible enhanced oil recovery fluid (or the insulating packer fluids of Leggett et al.) comprises a rheological additive, as noted above, added to a hydrocarbon fluid that includes one or more gelling agents, such as phosphoric acid esters in the presence of a ferric or aluminum compound. The hydrocarbons, for example, may be diesels, paraffin oils, crude oils, kerosene, or mixtures thereof. The phosphoric acid esters may have same or different alkyl groups, having various lengths. In accordance with embodiments of the invention, the alkyl groups (i.e., the ester parts) of the phosphoric acid esters have two or more carbon atoms, and preferably at least one of the alkyl groups has 3 to 10 carbon atoms. The ferric or aluminum compounds may be organic or inorganic compounds, such as aluminum chloride, aluminum alkoxide, ferric chloride, organometallic complexes of aluminum or iron(III), amine carboxylic acid salts of aluminum or iron(III), etc.

As identified in Leggett et al., the phosphoric acid esters having a desired alkyl group may be prepared using phosphorous pentaoxide and triethyl phosphate (TEP) (or other similar phosphate triesters) in the presence of a trace amount of water:

In the reactions shown above, the tri-ethyl phosphate ester (TEP) is partially hydrolyzed to produce a phosphoric acid diethyl ester. The phosphoric acid diethyl ester is then transesterified with a selected alcohol (ROH) to regenerate a phosphoric acid dialkyl ester having at least one and often two ester alkyl groups derived from the ROH.

As also taught in Leggett et al., the alcohol (ROH), i.e., the length of the alkyl chain R, may be selected to provide the desired hydrophobicity. In accordance with embodiments of the invention, the alcohols (ROH) have 2 or more carbons (i.e., ethanol or higher), and preferably, 2 to 10 carbons, which may be straight or branched chains. The phosphoric acid dialkyl esters having the alkyl chain of 2-10 carbons long may be obtained from M-I L.L.C. (Houston, Tex.) under the trade name of ECF-976. In accordance with some embodiments of the present invention, the R group may include aromatic or other functional groups, as long as it can still provide proper solubility in the hydrocarbon base fluids.

One of ordinary skill in the art would appreciate that various other reactions may be used to prepare the desired phosphoric esters without departing from the scope of the invention. For example, as noted in Leggett et al., phosphoric acid esters may be prepared using phosphorous hemipentaoxide (or phosphorous pentaoxide P₂O₅) and a mixture of long chain alcohols, as disclosed in U.S. Pat. No. 4,507,213:

This reaction produces a mixture of phosphoric acid monoesters and diesters. Furthermore, while the above reaction is shown with two different alcohols, the same reaction may also be performed with one kind of alcohol to simplify the product composition. Note that embodiments of the invention may use a mixture of phosphoric acid esters, i.e., not limited to the use of a pure phosphoric acid ester. As used herein, “phosphoric acid esters” include mono acid di-esters and di-acid monoesters. Furthermore, instead of or in addition to phosphoric esters, embodiments of the present invention may also use phosphonic acid esters, as disclosed in U.S. Pat. No. 6,511,944 issued to Taylor et al. A phosphonic acid ester has an alkyl group directly bonded to the phosphorous atom and includes one acid and one ester group.

A viscosified miscible enhanced oil recovery fluid in accordance with one embodiment of the invention, (like the insulating packer fluids of Leggett et al.) may be prepared as follows: a base fluid of hydrocarbons, a gelling agent comprising a phosphoric acid ester (e.g., ECF-976 product from M-I L.L.C.) or a phosphonic acid ester complexing with a multivalent metal ion (e.g., ferric or aluminum ion, or ECF-977 product from M-I L.L.C.), and a rheological additive (e.g., VersaPac™ alkyl diamides) are mixed in a blender (to shear the mixture) at an elevated temperature (e.g., 180° F., about 80° C.) to facilitate the dissolution or swelling of the dialkyl diamide. The base fluid may comprise, for example, diesels, a mixture of diesels and paraffin oil (e.g., 85%:15% mixture), mineral oil, IO 16/18 base fluid, Saraline 185V™ synthetic oil, or Safe-Solv OM™ (additive characterized as a combination of powerful, non-aromatic hydrocarbon and natural terpene solvents and surfactants with exceptional oil and grease solvent properties from M-I L.L.C.) and Safe-T-Pickle™ (additive characterized as a non-aromatic, high-flashpoint pipe dope solvent from M-I L.L.C.), EDC 99-DW™ (drilling fluid from TOTAL Special Fluids), or PureDrill HT-40™ (drilling mud base fluid from PetroCanada). In addition, a viscosified fluid in accordance with some embodiments of the invention may further comprise other components that are commonly used in such fluids, such as emulsifiers and inorganic salts (e.g., calcium chloride, calcium bromide, etc.). Examples of emulsifiers include those sold under the trade names of VersaMul™ and VersaCoat™ by M-I L.L.C. For example, a viscosified fluid of the invention may comprise a blend of diesel with about 3-9 ppb (pounds per barrel) Ecotrol RD™ (an oil soluble fluid control additive polymer from M-I L.L.C.) and about 3-9 ppb of the VersaPac™ product. One of ordinary skill in the art would appreciate that the gelling agents and the rheological additives may be added in a suitable amount for the desired properties.

As also taught in Leggett et al., since the VersaPac™ product (or similar alkyl diamides) is barely soluble in oil-based fluids, an alternative method of preparation involves first preparing a slurry (e.g., an 1:1 slurry) of VersaPac™ product in an appropriate solvent (e.g., propylene glycol, polypropylene glycol, or other similar solvents). This preparation may be performed with a blender at a lower temperature (e.g., 135° F., about 58° C.). This slurry is then added to the oil-based fluids and the gelling agents. Alternatively, instead of first preparing a slurry of VersaPac™ product in said appropriate solvent, the VersaPac™ product and then said appropriate solvent may simply be added to the oil-based fluids and the preparation may then be performed with a blender at a lower temperature (e.g., 135° F., about 58° C.). Then the gelling agent comprising a phosphoric acid ester (e.g., ECF-976 from M-I L.L.C.) or a phosphonic acid ester complexing with a multivalent metal ion (e.g., ferric or aluminum ion, or ECF-977 from M-I L.L.C.), is subsequently added to this mixture. And, as yet another alternative, instead of first preparing a slurry of VersaPac™ product in said appropriate solvent, the said appropriate solvent and then the VersaPac™ product may simply be added to the oil-based fluids and the preparation may then be performed with a blender at a lower temperature (e.g., 135° F., about 58° C.). Then the gelling agent comprising a phosphoric acid ester (e.g., ECF-976 from M-I L.L.C.) or a phosphonic acid ester complexing with a multivalent metal ion (e.g., ferric or aluminum ion, or ECF-977 from M-I L.L.C.), is subsequently added to this mixture. Of these three possible alternatives, the latter is slightly preferred over the other two; and all three of these alternatives (because they involve heating and shearing to only 135° F.) are slightly preferred over the alternative of adding all components at once and subjecting the mixture to heating and shearing to 180° F. In addition, it will be obvious to one skilled in the art that other methods may also be used to effect the same result.

When gelled hydrocarbon fracturing fluids (power-law fluids) or yield-power-law fluids are injected in accordance with the present invention, it may be desirable to practice alternative embodiments of the present invention wherein a mild acid such as, for example, acetic acid or the like, or an acid gas such as, for example, CO₂ or the like is injected prior to the miscible flooding. Some subterranean petroliferous formations include naturally occurring alkalinity which might adversely affect the rheological properties of said gelled hydrocarbon fracturing fluids or yield-power-law fluids; and the pre-injection of such an acid or acid gas will sacrifice the relatively inexpensive acid or acid gas to the neutralization of said alkalinity.

In one embodiment of the present invention, at the end of production of the original oil in place, subsequent steps are optionally performed such as, for example, employing a viscosity breaker to the injectant so that the larger part of the injectant may also be recovered from the formation in addition to virtually all of the original oil in place. For gelled hydrocarbon fracturing fluids or yield-power-law fluids injected in accordance with the present invention, examples of highly mobile breakers include NH₃ and the like.

As also taught in Leggett et al., FIG. 1 shows shear stress as a function of shear rate at 140° F. for a packer fluid. This FIG. 1 is also illustrative where the fluid is a viscosified miscible enhanced oil recovery fluid in accordance with one embodiment of the invention. As shown in FIG. 1, the data point fit a curve according to the “yield power law” equation (Herschel-Bulkley equation):

τ=τ₀ +K _(m)·{dot over (γ)}^(n) ^(m)

where τ is the shear stress (lb_(f)/100 ft²), τ₀ is the yield stress (lb_(f)/100 ft²), K_(m) is a consistency factor (which is equivalent to viscosity when τ₀ approaches 0 and n_(m) approaches 1.0, i.e., Newtonian behavior) (units are lb_(f)·sec^(n)/100 ft²), {dot over (γ)} is the shear strain rate (s⁻¹), and n_(m) is the flow behavior index, which is a unitless exponential parameter whose values typically range from 0.3 to 1.0. The curve fitting yields n_(m)=0.539, K_(m)=5.07 lb_(f)·sec^(n)/100 ft², and τ₀=12.70 lb_(f)/100 ft², which clearly shows a yield power law fluid. A Newtonian or simple power law fluid will have a zero yield stress value (τ₀).

As also in Leggett et al., advantages of the invention may include one or more of the following. Viscosified miscible enhanced oil recovery fluids in accordance with embodiments of the invention have yield power law characteristics such that they are not prone to movement once they are emplaced in an annulus. Minimization of movements in these fluids reduces convective heat loss to a minimum. These yield power law fluids can still be pumped during emplacement and displacement. The base fluids may be selected from various hydrocarbons such that they will have inherently low thermal conductivity and suit particular applications, e.g., deepwater or Arctic/Antarctic areas.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. A viscosified miscible enhanced oil recovery fluid for injection into a subterranean petroliferous formation comprising: a weighted hydrocarbon-based fluid comprising a nano-scale weighting agent dispersed in a hydrocarbon-based fluid, and at least one additive selected from the group consisting of viscosifying agents, gelling agents and rheological agents, wherein said viscosified miscible enhanced oil recovery fluid has fluid behavior characteristics selected from the group consisting of yield power law fluids, power law fluids, Bingham-plastic fluids and Newtonian fluids.
 2. The viscosified fluid of claim 1, wherein said nano-scale weighting agent comprises at least one of barium sulfate, barium carbonate, zinc oxide, zinc nitride, magnesium oxide, or iron oxide.
 3. The viscosified fluid of claim 1, wherein said nano-scale weighting agent comprises a nano-scale-zinc-oxide.
 4. The viscosified fluid of claim 1, wherein said gelling agent comprises a multivalent metal ion and at least one ester selected from the group consisting of a phosphoric acid ester and a phosphonic acid ester.
 5. The viscosified fluid of claim 4, wherein said multivalent metal ion is at least one selected from the group consisting of a ferric ion, an aluminum ion, a chelated ferric ion and a chelated aluminum ion.
 6. The viscosified fluid of claim 1, wherein said rheological agent is an alkyl diamide having a formula: R₁—HN—CO—(CH₂)_(n)—CO—NH—R₂, wherein n is an integer from 1 to 20, R₁ is an alkyl groups having from 1 to 20 carbons, and R₂ is hydrogen or an alkyl group having from 1 to 20 carbons.
 7. The viscosified fluid of claim 1, wherein said rheological agent is present at a concentration of 3-13 pounds per barrel.
 8. The viscosified fluid of claim 1, further comprising a solvent for said rheological agent.
 9. The viscosified fluid of claim 1, wherein said hydrocarbon-based fluid comprises at least one selected from diesel, a mixture of diesels and paraffin oil, mineral oil, and isomerized olefins.
 10. The viscosified fluid of claim 1, wherein said at least one additive comprises a viscosifying agent.
 11. The viscosified fluid of claim 1, wherein said at least one additive comprises a gelling agent.
 12. The viscosified fluid of claim 1, wherein said at least one additive comprises a gelling agent and a rheological agent.
 13. The viscosified fluid of claim 1, wherein said at least one additive comprises a gelling agent, a rheological agent and a solvent for said rheological agent.
 14. A method for preparing a viscosified miscible enhanced oil recovery fluid for injection into a subterranean petroliferous formation comprising the steps of: preparing a mixture of a weighted hydrocarbon-based fluid comprising a nano-scale weighting agent dispersed in a hydrocarbon-based fluid and at least one additive selected from the group consisting of viscosifying agents, gelling agents and rheological agents; optionally heating said mixture to a selected temperature; and optionally shearing said mixture, wherein said mixture has fluid behavior characteristics selected from the group consisting of yield power law fluids, power law fluids, Bingham-plastic fluids and Newtonian fluids.
 15. A method for enhanced oil recovery from a subterranean petroliferous formation comprising the steps of: preparing a viscosified miscible enhanced oil recovery fluid comprising: a mixture of (a) a weighted hydrocarbon-based fluid comprising a nano-scale weighting agent dispersed in said hydrocarbon-based fluid, and (b) at least one additive selected from the group consisting of viscosifying agents, gelling agents and rheological agents; and pumping said fluid mixture from the surface into said subterranean petroliferous formation; wherein said viscosified miscible enhanced oil recovery fluid exhibits fluid behavior characteristics selected from the group consisting of yield power law fluid characteristics, power law fluid characteristics, Bingham-plastic fluid characteristics, and Newtonian fluid characteristics.
 16. The method of claim 15, wherein said nano-scale weighting agent comprises at least one of barium sulfate, barium carbonate, zinc oxide, zinc nitride, magnesium oxide, or iron oxide.
 17. The method of claim 15, wherein said nano-scale weighting agent comprises a nano-scale-zinc-oxide.
 18. The method of claim 15, wherein said gelling agent comprises a multivalent metal ion and at least one ester selected from the group consisting of a phosphoric acid ester and a phosphonic acid ester.
 19. The method of claim 18, wherein said multivalent metal ion is at least one selected from the group consisting of a ferric ion, an aluminum ion, a chelated ferric ion and a chelated aluminum ion.
 20. The method of claim 15, wherein said rheological additive is an alkyl diamide having a formula: R₁—HN—CO—(CH₂)_(n)—CO—NH—R₂, wherein n is an integer from 1 to 20, R₁ is an alkyl groups having from 1 to 20 carbons, and R₂ is hydrogen or an alkyl group having from 1 to 20 carbons.
 21. The method of claim 15, wherein said rheological additive is present at a concentration of 3-13 pounds per barrel.
 22. The method of claim 15, further comprising a solvent for said rheological agent.
 23. The method of claim 15, wherein said hydrocarbon fluid comprises at least one selected from diesel, a mixture of diesels and paraffin oil, mineral oil, and isomerized olefins.
 24. The method of claim 15, wherein said at least one additive comprises a viscosifying agent.
 25. The method of claim 15, wherein said at least one additive comprises a gelling agent.
 26. The method of claim 15, wherein said at least one additive comprises a gelling agent and a rheological agent.
 27. The method of claim 15, wherein said at least one additive comprises a gelling agent, a rheological agent and a solvent for said rheological agent.
 28. The method of claim 15 further comprising the step of subsequently recovering some of said viscosified miscible enhanced oil recovery fluid back to the surface from said subterranean petroliferous formation.
 29. The method of claim 15 further comprising the step of subsequently recovering some of said viscosified miscible enhanced oil recovery fluid by (a) applying a viscosity breaker proximate said viscosified miscible enhanced oil recovery fluid in said subterranean petroliferous formation and (b) flowing said viscosity-broken fluid back to the surface.
 30. The method of claim 15 further comprising the initial step of determining whether alkalinity conditions exist in said petroliferous formation that could be damaging to said viscosified miscible enhanced oil recovery fluid, and if so, prior to the injection of said miscible viscosified enhanced oil recovery fluid, injecting a mild acid or acid gas to neutralize said alkalinity. 